Fat-cat freakout

So the oddest thing happened last night. The LWI and I were downstairs, relaxing on the couch after dinner, when we heard some sort of noise from upstairs - afterwards we couldn't remember just what it was, but we knew we'd both registered hearing something, like a faint thump or other indeterminate noise. That was immediately followed by two top-volume blood-curdling yowls from TwoCat. We were startled, but assumed that he was just getting the worst end of a fight with OneCat - they play-fight often, and once in a while one of them yells that the other is taking the fight a little too seriously. But a half-second later we realized that OneCat was with us in the living room, looking just as startled as we were.

So I headed upstairs to see what TwoCat had gotten himself into, and found him at the top of the stairs, all puffed up and bug-eyed and spooked. TwoCat is the mildest of creatures, and even all puffed up he doesn't look very intimidating, more like a black furry basketball, but he was more worked up than I'd ever seen him. I took another step up the stairs to see what was the matter, and he flinched like he was ready to bolt, so for the next few minutes I eased my way up the stairs to soothe him and figure out what he'd done to himself. At this point I was thinking perhaps he'd jumped down off a chair or desk and landed funny, twisting a leg or a paw; he's ordinarily a stoic little guy, and doesn't make much of a fuss about anything, but that full-throated howl was still echoing in my ears. But after a while he calmed down and bounded downstairs to see if there was any dinner in his bowl, and showed no signs of physical damage at all. WTF? I canvassed the upstairs, and found no evidence of anything out of the ordinary - nothing tipped over, no giant rats to fight with, no bloodstains, nothing that looked at all like it was worth screaming about.

We remain perplexed.